That's because at least 70 percent of all cellphone text spam is designed to defraud you in some way, according to a study by Cloudmark, a company that makes anti-spam software. In contrast, only about 10 percent of spam arriving by email is sent with that intent.

Spam in text messages may try to guide you to shady websites that will install "malware" on your phone to vacuum up all the personal data stored in it. Or the messages may urge you to dial a phone number where your personal and financial information is solicited. Whatever you provide is either used to steal your identity or sold to third parties who'll send you yet more spam texts.

In 2012, cellphone spam — often called "smishing " — has tripled compared with 2011 levels, while email-delivered spam continues to drop, following an 82 percent decrease last year.

Why? For scammers, cellphone text messages are fast, cheap, easy and effective. The thieves use inexpensive, hard-to-track prepaid phones to transmit myriad text come-ons. When word spreads of the spam du jour, the scammers simply toss the phones they're using and buy new ones.

What's more, cellphone users are three times more likely than computer users to respond to spam. "Because you always have your cellphone with you, answering it becomes more of an impulse reaction," says Cloudmark security researcher Mary Landesman.

• Ignore instructions to text "STOP" or "NO" to prevent future texts. This is a common ploy by scammers to confirm they have a live, active contact for more cellphone spam, says Landesman. Never dial call-back numbers either.

• Forward the texts to 7726 (SPAM on most keypads). This will alert your cellphone carrier to block future texts from those numbers.

• Anti-malware software is available for many phones. The trade-off may be reduced battery life, so check with your service provider or phone manufacturer for recommendations.

• Install upgrades to your security software. If you suspect an upgrade notification is phony, check with your cellphone or software provider.

• Never store credit card or account log-in information in emails or notes on your cellphone.

• When you get a text promising you a $1,000 gift card, ask yourself: Would anyone really give me that? Know, too, that banks and other legitimate businesses don't send customers unsolicited texts.

Sid Kirchheimer is the author of Scam-Proof Your Life, published by AARP Books/Sterling.